I voted for whenever necessary because that's been the way I've done it all through T & P's lives. When they were young, I took them in for their kitten shots and to be fixed, but once they were done with those and they were, oh say about 1 1/2 - 2 years old, I only took them in when necessary. They have both been indoor kitties their entire lives so I didn't think it was really necessary to get all the shots all the time. When I moved to Chicago I took them in one last time before I left home because I didn't know how things would go in Chicago and if I'd be able to pay for vet bills and such, but I was lucky and they never needed any medical care whatsoever...until they got old. Now Tubby goes in every 6 months so they can check his kidney levels, and it turns out I'll have to take Peanut in every 6 months to keep an eye on her thyroid. So you see, event though they are going more often these days, it's still "whenever necessary" because in my opinion, these 6 month checkups are necessary.